A rolling bearing unit is used to rotatably support the wheel of an automobile with respect to a suspension system. Moreover, in order to control an anti-lock brake system (ABS) or a traction control system (TCS), it is necessary to detect the rotational speed of the wheel. Therefore, a method has been widely used recently where the wheel is rotatably supported with respect to the suspension system, and the rotational speed of the wheel is detected, by using a rolling bearing unit with a rotational speed detecting device in which a rotational speed detecting device is incorporated into the rolling bearing unit.
As one example of a rolling bearing unit with a rotational speed detecting device used for such a purpose, a structure as shown in FIGS. 5 and 6 is disclosed in Patent Reference 1 (Japanese Patent Application Publication No. H09-21822). This conventionally-known rolling bearing unit with a rotational speed detecting device rotatably supports a hub 4 comprising a hub body 2 and an inner ring 3 such that it can rotate freely on the inner diameter side of an outer ring 1 which is supported and fixed to a suspension system (not shown in the figure), and which does not rotate during use. The outer ring 1 has an external flange shaped attachment portion 5 on its outer peripheral surface, and double-row outer ring raceways 6 on its inner peripheral surface. The hub body 2 has a flange 7 for supporting the wheels on the outside (relative to the axial direction, the outer side in the vehicle widthwise direction when assembled on an automobile. Similarly in the present specification. The left side in the figures excluding FIG. 6) end portion of the outer peripheral surface. Furthermore, a first inner ring raceway 8 is formed on a middle portion of the outer peripheral surface of the hub body 2, and the inner ring 3 formed with a second inner ring raceway 10 on its outer peripheral surface, is fitted onto a small diameter step portion 9 formed on the inside (relative to the axial direction, on the vehicle widthwise central side when assembled on an automobile. Similarly in the present specification. The right side of the figures excluding FIG. 6) end portion.
Moreover, between the outer ring raceways 6 and the inner ring raceways 8 and 10, a plurality of rolling elements 11 are rotatably provided, and the hub 4 is supported on the inner diameter side of the outer ring 1 such that it rotates freely. Furthermore, a first seal ring 12 is provided between the outside opening of the outer ring 1 and the outer peripheral surface of the middle portion of the hub body 2. This first seal ring 12 closes the outside opening of a cylindrical space 13 in which the rolling elements 11 are provided, in order to prevent grease in the space 13 from leaking outside, or external foreign substances from entering the space 13. Here, in the example of the figure, balls are used as the rolling elements 11. However, in the case of a rolling bearing unit with a rotational speed detecting device for an automobile, whose weight is higher, tapered rollers may be used as the rolling elements. Furthermore, a drive shaft 15 is provided in a central hole 14 of the hub body 2 such that it can be inserted freely, and the outer peripheral surface of the drive shaft 15 and the inner peripheral surface of the central hole 14 are engaged freely using a spline. When the automobile is traveling, the hub body 2 is rotated by the drive shaft 15 via a constant velocity joint 16 provided on its inside end.
On the other hand, a combination seal ring 17 is provided between the outer peripheral surface of a portion at the inside end of the inner ring 3, which is inwardly away from the second inner ring raceway 10, and the inner peripheral surface of the inside end of the outer ring 1, so that the inside opening of the space 13 is closed. Half of the outer diameter side of a metal core 19 of an inner diameter side seal ring element 18, which constitutes the combination seal ring 17, is formed in a comb shape, and the magnetic characteristics on the inner surface of the half of the outer diameter side are changed alternately around the direction of the circumference at equal spacing, thus providing a function as an encoder in that half of the outer diameter side of the metal core 19.
On the other hand, a cover 20, whose cross-section is approximately L-shaped, and which is formed in a circular shape overall by forming from a metal plate using a press, is externally secured on the outer peripheral surface of the inside end of the outer ring 1. A rotation-detecting sensor 21 is supported in the inside of the cover 20. The rotation-detecting sensor 21 is encapsulated using a synthetic resin 22, and its detecting section faces the inside surface of the half of the outer diameter side of the metal core 19 that functions as the encoder. The rotation-detecting sensor 21 is formed by a permanent magnet 23, which is polarized in the axial direction (horizontal direction of FIG. 5), and a magnetic detecting element 24 such as a Hall IC, a magnetoresistive element or the like. A signal from the rotation-detecting sensor 21 is transmitted via a harness 25 to a controller (not shown in the figure), which is provided on the vehicle body side, in order to control the ABS and TCS.
A seal ring 26 is fitted around the whole circumference of the inner peripheral surface of the inside end of the cover 20. The arrangement is such that the edge of the seal ring 26 makes a sliding contact all around the circumference of the surface of the constant velocity joint 16 when the drive shaft 15 is inserted into the central hole 14 of the hub body 2.
According to the rolling bearing unit with a rotational speed detecting device disclosed in Patent Reference 1, which is constructed as described above, a wheel fixed onto the hub body 2 can be rotatably supported on the suspension system supporting the outer ring 1. Furthermore, when the metal core 19 which functions as an encoder rotates together with the inner ring 3 fixed on the inside end of the hub body 2, accompanying rotation of the wheel, the density of the magnetic flux transmitted to the magnetic detecting element 24 of the rotation-detecting sensor 21 changes. Hence the resistance value or the output voltage of the rotation-detecting sensor 21 change. In this manner, the frequency at which the value of the resistance or the output voltage of the rotation-detecting sensor 21 changes is proportional to the rotational speed of the wheel. Therefore, if an output signal from the rotation-detecting sensor 21 is input to the controller (not shown in the figure), it is possible to obtain the rotational speed of the wheel, and control the ABS and TCS appropriately.
Moreover, in the case of the conventional structure shown in FIGS. 5 and 6, the seal ring 26 is fitted to the inside end of the inner peripheral surface of the cover 20, and the edge of the seal ring 26 makes a sliding contact all around the circumference of the surface of the constant velocity joint 16. Therefore when the rolling bearing unit with a rotational speed detecting device is used, it is possible to prevent foreign substances such as magnetic powder and the like from entering the cover 20. Accordingly, it is possible to prevent foreign substances such as magnetic powder and the like from sticking near the permanent magnet 23 of the rotation-detecting sensor 21, and to prevent the accuracy of the detected rotational speed of the wheel from deteriorating.
In the case of the conventional structure as shown in FIGS. 5 and 6, the metal core 19, which is a structural element of the combination seal ring 17 for closing the inside opening of the space 13 in which the rolling elements 11 are provided, has a function as an encoder. However, the rotation-detecting sensor 21 is supported in the cover 20, which is independent from the combination seal ring 17. Therefore, not only is it difficult to miniaturize the rotational speed detecting device section, but also the assembly operation of the rotational speed detecting device section is troublesome. To address this, Patent Reference 2 (Japanese Patent Application Publication No. H3-279061), Patent Reference 3 (U.S. Pat. No. 6,559,633), and Patent Reference 4 (U.S. Pat. No. 6,499,885), disclose a structure of a rolling bearing unit with a rotational speed detecting device, in which not only an encoder but also a rotation-detecting sensor are installed in a combination sealing section.
FIG. 7 shows a rolling bearing unit with a rotational speed detecting device as disclosed in Patent Reference 4. In the case of this second example of a conventional structure, a circular case 27 made of synthetic resin is fitted into the inside end of an outer ring 1, and a combination seal ring 17 is provided between the inner peripheral surface of the case 27 and the outer peripheral surface of the inside end of the inner ring 3. Of the inner diameter side and the outer diameter side seal ring elements 18 and 28, which constitute the combination seal ring 17, the inner diameter side seal ring element 18 is externally fixed on the inside end of the inner ring 3. On the other hand, the outer diameter side seal ring element 28 is connected and fixed on an inner peripheral surface portion of the case 27 at the same time as injection molding the case 27. Furthermore, an annular encoder 29 is fixed to the inner diameter side seal ring element 18, and a rotation-detecting sensor 21a including a magnetic detecting element 24a is fixed to the outer diameter side seal ring element 28.
According to the structures disclosed in Patent References 2 to 4, including the second example of an existing structure shown in FIG. 7, it is possible to miniaturize the rotational speed detecting device section, and to simplify the assembly operation of the rotational speed detecting device section. However, the structure disclosed in Patent Reference 2 requires extremely small sensors and encoders when it is actually used for a rolling bearing unit for an automobile. Accordingly, it is not capable of practical use with current technical standards.
On the other hand, in the case of the invention disclosed in Patent References 3 and 4, the sensor and encoder disclosed have realistic dimensions. However, in the structure disclosed in Patent Reference 3 an encoder is supported on the inside surface of an annular portion of a slinger externally fixed to the inside end of a hub, and the edge of the seal member makes a sliding contact with the outer peripheral surface of the cylinder section of the slinger, and a part of the inside surface of the annular portion, which is toward the outer diameter. In the case of such a structure disclosed in Patent Reference 3, the design is not realistic as a combination seal ring. For instance the width of the part with which the edge of the seal member makes a sliding contact is small, and so forth. As a result, in the case where the outer ring and the hub are displaced due to the load applied to the rolling bearing unit during use, the edge of the seal member comes away from the mating surface, so there is a possibility that the necessary sealing performance cannot be ensured.
Moreover, in the case of the structure disclosed in Patent Reference 4, in addition to a metal core 30 constituting the outer diameter seal ring element 28 there is a seal member 31 constituting the inner diameter side seal ring element 18, between the surface to be detected of the encoder 29, and the detecting section of the magnetic detecting element 24a. As a result, it is not possible to make the distance between the surface to be detected and the detecting section small. In order to increase the reliability of the rotational speed detection, it is preferable to make this distance small. Therefore, a structure in which the distance cannot be made small is not desirable.
Furthermore, in the case of the structure disclosed in Patent Reference 4, since the outer diameter side seal ring element 28, which supports the rotation-detecting sensor 21a, is internally fixed to the outer ring 1 via the case 27 made of synthetic resin, it is difficult to ensure the strength of the support of the rotation-detecting sensor 21a. That is, the coefficient of friction that acts on the contact portion between the outer peripheral surface of the case 27 made of synthetic resin, and the inner peripheral surface of the outer ring 1 made of metal, is small. Hence the case 27 can easily rotate relative to the outer ring 1 with comparatively little force. Moreover, due to the difference in the coefficient of expansion between synthetic resin and metal, there is a possibility that the surface pressure of the contact portion is reduced. The reduction in the surface pressure of the contact portion occurs directly at low temperature, and also due to permanent deformation in fatigue of the synthetic resin, which accompanies an excessive increase in surface pressure at high temperature. For whichever reason, when the surface pressure of the contact portion is reduced, coupled with the fact that the coefficient of friction of the contact portion is low, the case 27 rotates relative to the outer ring 1. When it rotates, it causes a failure such as disconnection of the harness 25 for obtaining a signal from the rotation-detecting sensor 21a. 
[Patent Reference 1] Japanese Patent Application Publication No. H9-21822
[Patent Reference 2] Japanese Patent Application Publication No. H3-279061
[Patent Reference 3] U.S. Pat. No. 6,559,633
[Patent Reference 4] U.S. Pat. No. 6,499,885